wikipedia|Red Letter Days
From the practice of marking feast-days in red on church calendars.
red letter day
- idiom A particularly significant day (personal or sectarian,) usually very positive, sometimes very negative.
#:Monday was a red letter day for her. She accomplished a lot and had fun doing it.
#:We saw losses for the days in a row, but w:Wall Street Crash of 1929|Black Tuesday was the worst red letter day of them all.
#:1975, John Ankenbruck, Twentieth Century History of Fort Wayne - Page 286
#::"In the meantime, the St. Valentine's Day Massacre at Chicago was a red-letter day in the gangster wars."